Geochemistry of evaporite in the Red Sea

One of the major shipboard findings during Leg 23 drilling in the Red Sea was the presence of late Miocene evaporites at Sites 225, 227, and 228. The top of the evaporite sequence correlates with a strong reflector (Reflector S) which has been mapped over much of the Red Sea (Ross et al., 1969, Phillips and Ross, 1970). This indicates that the Red Sea appears to be extent. Miocene sediments, including evaporites, are known from a few outcrops along the coastal plains of the Gulf of Suez to lat 14°N (Sadek, 1959, cited in Friedman, 1972; Heybroek, 1965; Friedman, 1972). Along the length of the Red Sea, the presence of Miocene salt is indicated by seismic reflection studies (Lowell and Genik, 1972) and confirmed by drilling. The recently published data from deep exploratory wells (Ahmed, 1972) demonstrate the great thickness of elastics and evaporites which were deposited in the Red Sea depression during Miocene time. The Red Sea evaporites are of the same age as the evaporites found by deep sea drilling (DSDP Leg 13) in the Mediterranean Sea. Therefore, Reflector S in the Red Sea is comparable to Reflector M in the Mediterranean. It is assumed that during Miocene time a connection between these two basins was established (Coleman, this volume) resulting in a similar origin for the evaporites deposited in the Red Sea and in the Mediterranean Sea. The origin of the Mediterranean evaporites has been discussed in great detail (Hsü et al., 1973; Nesteroff, 1973; Friedman, 1973). The formation of evaporites may be interpreted by three different hypotheses. 1) Evaporation of a shallow restricted shelf sea or lagoon which receives inflows from the open ocean. 2) Evaporation of a deep-water basin which is separated from the open ocean by a shallow sill (Schmalz, 1969). 3) Evaporation of playas or salt lakes which are situated in desiccated deep basins isolated from the open ocean (Hsü et al., 1973). The purpose of this study is to show whether one of these models might apply to the formation and deposition of the Red Sea evaporites. Therefore, a detailed petrographic and geochemical investigation was carried out.

(Table 1) Compressional- and share-wave velocieties at elevated confining pressure of ODP Holes 136-842C and 136-843A

Identification of a sediment/basement contact using seismic reflection recordings has proven to be extremely difficult in wide areas of the North Pacific Ocean owing to the presence of massive, highly reflective chert layers within the sediment column. Leg 136 of the Ocean Drilling Program recovered coherent pieces of chert of sufficient size for the first comprehensive laboratory measurements of the seismic properties of this material. Compressional-wave velocities of six samples at 40-MPa confining pressure averaged 5.33 km/s, whereas shear-wave velocities at the same pressure averaged 3.48 km/s. Velocities were independent of porosity, which ranged from 5% to 13%, suggesting that pores within the samples were mostly high aspect ratio vugs as opposed to low aspect ratio cracks. Back-scattered electron images made with a scanning electron microscope confirmed this observation. Acoustic impedances were calculated for the chert samples and from shipboard measurements of the red clay sediment overlying the chert layers. An extremely large compressional-wave reflection coefficient (0.73) characterized the interface between the two lithologies. A synthetic seismogram was calculated using chert and typical pelagic carbonate properties to illustrate the influence of chert layers on a marine seismic-reflection section. Compressional-wave to shear-wave velocity ratios of the chert samples (Vp/Vs =1.53) are close to that of single-crystal quartz in spite of variable porosity. Shear-wave reflection coefficients are estimated to be approximately 0.94. A compressional-wave reflection coefficient for a basement/sediment (carbonate) interface is estimated to be approximately 0.50, significantly less than that of sediment/chert.

Chemical versus structural defense against fish predation in two dominant soft coral species (Xeniidae) in the Red Sea

Soft corals of the family Xeniidae are particularly abundant in Red Sea coral reefs. Their success may be partly due to a strong defense mechanism against fish predation. To test this, we conducted field and aquarium experiments in which we assessed the antifeeding effect of secondary metabolites of 2 common xeniid species, Ovabunda crenata and Heteroxenia ghardaqensis. In the field experiment, the metabolites of both investigated species reduced feeding on experimental food pellets in the natural population of Red Sea reef fishes by 86 and 92% for O. crenata and H. ghardaqensis, respectively. In the aquarium experiment, natural concentration of crude extract reduced feeding on experimental food pellets in the common reef fish Thalassoma lunare (moon wrasse) by 83 and 85%, respectively. Moon wrasse feeding was even reduced at extract concentrations as low as 12.5% of the natural concentration in living soft coral tissues. To assess the potential of a structural anti-feeding defence, sclerites of O. crenata were extracted and mixed into food pellets at natural, doubled and reduced concentration without and in combination with crude extract at 25% of natural concentration, and tested in an aquarium experiment. The sclerites did not show any effect on the feeding behavior of the moon wrasse indicating that sclerites provide structural support rather than antifeeding defense. H. ghardaqensis lacks sclerites. We conclude that the conspicuous abundance of xeniid soft coral species in the Red Sea is likely a consequence of a strong chemical defence, rather than physical defences, against potential predators.

(Table 1) Clay mineralogy at DSDP Sites 86-576 and 86-578

Late Cretaceous and Cenozoic sediments deposited on the eastern and western sides of the Shatsky Rise show similar clay assemblages of chlorite, illite, mixed layers, smectite, kaolinite, and palygorskite, and a similar vertical differentiation into three units. The deep-sea pelagic red clay is dominated by terrigenous components, which suggests that diagenesis is minor in this part of the western North Pacific and that eolian supply is very important. The role of the Shatsky Rise as a submarine barrier is of minor importance. The volcanic contribution to the clay component appears very small, as in some other parts of the Pacific Ocean. Stratigraphic correlations between Sites 576 and 578, which can be made from mineralogical data, partly offset the rareness of biostratigraphic markers.